The synthesis and characterization of a series of cobalt(II) complexes which contain coordinated pentadentate ketoiminato and thioiminato ligands will be completed, and their reactivity toward molecular oxygen will be investigated. Visible spectral and EPR measurements will be performed in order to determine the thermodynamics associated with oxygenation and the structures of the oxygen adducts which form. Differences in Co-O2 bond stabilities will be correlated with different electronic and structural features of the ligands. In particular, the relationship between oxygen adduct stability and the presence of trans vs. cis sulfur atoms will be studied. We believe that such results are necessary in order to determine the relative importance of pibonding vs. polarization effects in the thio Schiff base systems. Additional studies to be persued include (1) incorporation of bulky substituents at selected sites on the iminato chelate rings in an effort to isolate crystalline adducts which are stable at room temperature, (b) investigation of the oxygen reactivity of several macrocyclic cobalt(II) complexes, and (c) continuation of our studies related to the synthesis and characterization of thioiminato molybdenum complexes and their reactions with small covalent molecules.